have actually useful error messages

[bmorel]

Neither of those are:

• execvp failed: No such file or directory errno = 2
• Failed to start Unvanquished process.

Even worst, "Failed to start" is the only one a user not using a terminal would see. The name of the file should be provided.
FWIW this happened after I tried to clean the game entirely, removing all the files. Updater thinks the game is installed but can't find any file, related to #109. Basically, updater tries to open daemon in Unvanquished folder, while said folder does not exists (and thus, neither do daemon obviously)